I just live by the philosophy that any computer I buy will be pretty much obsolete by the time I get it home, and that works for me. 🙂
Seriously, though, if the computer still does what you ask of it, it really isn't obsolete. If it takes a few extra seconds to do something in Photoshop, is it really obsolete? Like some other folks have said, I tend to stretch as much life as possible from my machines. My 3+ year old Powerbook is still going along fairly well, and the 2 year old Mini does everything I ask of it.